The invention relates to a system for converting DC to AC which involves a plurality of inverters, for single phase as well as for multiphase AC output. The invention is applicable to variable frequency inverter motor drives, in particular to high voltage and high power rating motor drives. In this respect, the invention also provides for a control system for such multi-inverter type of DC/AC converter. More particularly, the invention relates to GTO or transistor inverter-type of DC/AC converter systems.
It is known to make a high-voltage thyristor converter by serially connecting thyristor units and to provide separate gate drive units for individual groups of thyristors to serially connect the groups being connected in a chain. See for instance U.S. Pat. No. 4,084,221.
Single pole inverters are known which have received various applications. See for instance U.S. Pat. Nos. 3,636,430; 4,173,779; 4,220,896.
It is known also to group single-pole inverters so as to form a three-phase AC output from a single DC source. See for instance U.S. Pat. Nos. 3,622,863 and 3,775,663.
Low voltage (460 V) variable frequency inverters for motor drives have been developed using inverter "poles" which include switching devices such as GTOs with 1200 V-blocking capability. To extend these inverters for higher voltage (e.g., 4160 V) and higher power rating, it is known to add the power output of various low-voltage inverters by means of output transformers. Thus, in a high-voltage input system, the power is transformed down to a suitable low voltage for the inverters and then transformed back to a high voltage by means of one or more output transformers. An input isolation transformer is normally expected in high-voltage systems; however, the need for an output transformer represents an additional and undesirable cost. This drawback is aggravated because the output transformer has to be capable of increased volts/Hz at low frequency output in order to satisfy the "boost" voltage requirements of motor loads.
Schemes have been proposed to eliminate the output transformer altogether with the design of high-voltage inverter "poles" in which the switching devices are series-connected. This approach is technically feasible but has the disadvantage of requiring some device derating and selection to ensure sharing of voltage across the series string of devices under switching conditions.
The technique proposed according to the present invention eliminates the use of an output transformer for high-voltage motor drives. It ensures good voltage-sharing among a number of low-voltage devices, without the need for special device selection. Moreover, it offers opportunities for multipulse output connections and it simplifies voltage control.
The object of the present invention is to use low-voltage inverters and to combine them so as to achieve higher voltage at the output. Another object of the invention with such a high-voltage multi-inverter system is to allow switching characteristics compatible with the high voltage applications of such a system, for instance load-sharing. A further object of the present invention is to provide a multi-inverter system of flexible design which is modular in nature and adapted to modular design for specific applications, in particular by the selection of a particular grouping of modular units, together with global and refined adjustment of the overall high-voltage characteristic of the system.